<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        #dv{width: 300px;height: 200px;background: red;}
    </style>
</head>
<body>
    <button id="btn1">隐藏</button>
    <button id="btn2">显示</button>
    <div id="dv"></div>
    <ol>
        <li>d</li>
        <li>d</li>
        <li>d</li>
        <li>d</li>
        <li>d</li>
    </ol>
    <script>
        document.getElementById('btn1').onclick=function(){
            alert("1")
            // document.getElementById('dv').style.color='blue';
            document.getElementById("dv").style.color="blue";
        }
    </script>
</body>
</html>